This invention relates to a method of correcting gradation for use with image display apparatus such as a CRT (cathode-ray tube), an LCD (liquid-crystal display) and an LP (laser printer). The invention also relates to an image display system employing such method of gradation correction.
Image display apparatus such as CRT, LCD and LP are currently used in various fields and image display systems incorporating such image display apparatus are under development. Digital image display apparatus need gradation correction and in order to ensure that they eventually provide the intended display characteristics, a dedicated look-up table (LUT) is constructed for gradation correction. To construct it, the gradation characteristics inherent in each image display apparatus are examined and compared with the input of the desired (or aimed) gradation characteristics. An image is processed with the thus constructed LUT and presented on the image display apparatus of interest.
The inherent characteristics of image display apparatus such as CRT and LCD and film/light box (Schaukasten) systems typically adapted to display X-ray images are measured by luminance and the obtained luminance values are usually input to construct a LUT for gradation correction. If a curve representing the desired gradation that is aimed at a target (hereunder referred to as a xe2x80x9cdesired gradation curvexe2x80x9d) is defined in terms of the relationship between luminance and data value, both the inherent and desired characteristics are expressed by the relationship between luminance and data value and it is possible to construct a smooth correction LUT.
However, if a desired gradation curve is defined in terms of data which is not the luminance data such as the logarithm of luminance [log(brightness)] and data value, a smooth correction LUT is not obtained. In most cases, both the inherent characteristic curve which represents the inherent characteristics of a specific apparatus and the desired gradation curve are obtained by constructing continuous data through interpolation of discrete data of measurement. A problem with this approach is that given a small number of discrete points, simple linear interpolation does not suffice to assure smoothness for the desired gradation curve, inherent characteristic curve and correction LUT. If the correction LUT is not smooth enough, discontinuities occur in the gradation of a displayed image, producing artifacts in the image. This can be a serious problem in the medical field, in which high-quality images are particularly needed and artifacts are a potential cause of erroneous diagnoses.
The present invention has been accomplished under these circumstances and has as an object providing a method of gradation correction that is capable of producing a smooth enough correction LUT to allow gradation correction to be performed in such a way that an image display apparatus will eventually provide the intended display characteristics.
Another object of the invention is to provide an image display system employing this method of gradation correction and being capable of displaying an image of high quality that has no defects such as artifacts.
The first object of the present invention is attained by its first and second aspect. According to its first aspect, the present invention provides a method of gradation correction of input image data to be displayed on an image display apparatus, comprising the steps of acquiring first characteristic values which represent the display characteristics inherent in the image display apparatus, converting the first characteristic values to logarithmic data, acquiring as the logarithmic data second characteristic values which represent the desired gradation to be eventually realized with the image display apparatus, constructing a gradation correction table based on the logarithmic data of the first characteristic values and the logarithmic data of the second characteristic values that have been converted logarithmically, and performing gradation correction on the input image data to the image display apparatus using the gradation correction table.
In the first aspect of the invention, the logarithmic data of the second characteristic values are preferably acquired by inputting directly the logarithmic data of the second characteristic data or by inputting the second characteristic data and then converting the second characteristic data to the logarithmic data.
According to its second aspect, the present invention provides a method of gradation correction further comprising the step of interpolating the logarithmic data of the first characteristic values and the logarithmic data of the second characteristic values that have been converted logarithmically in the first aspect of the prevent invention, wherein the gradation correction table is constructed based on the logarithmic data of the first characteristic values and the logarithmic data of the second characteristic values.
In both the first and second aspects of the invention, the interpolation is preferably selectable from between high-order interpolation and linear interpolation.
It is also preferred that the interpolation is adapted to be switched between high-order interpolation and linear interpolation and vice versa depending upon a number of discrete points to be interpolated.
The second object of the present invention can be attained by its third and fourth aspects. According to its third aspect, the present invention provides an image display system, comprising an image display apparatus on which displays input image data, a first characteristic value acquiring device which acquires first characteristic values which represent display characteristics inherent in the image display apparatus, a first logarithmic conversion device which converts the first characteristic values to logarithmic data, a logarithmic data acquiring device which acquires logarithmic data of second characteristic values which represent the desired gradation to be eventually realized with the display apparatus, a gradation correction table constructing device by which a gradation correction table is constructed based on the respective logarithmic data of the first characteristic values and the second characteristic values that have been converted logarithmically, and a gradation correction device which performs gradation correction on the input image data using the gradation correction table wherein the corrected input image data on which the gradation correction is performed by the gradation correction device is displayed on the image display apparatus.
In the third aspect of the invention, the logarithmic data acquiring device is a direct input device which inputs directly the logarithmic data of the second characteristic values.
It is also preferred that the logarithmic data acquiring device comprises an input device which inputs the second characteristic values and a second logarithmic conversion device which converts the second characteristic values to the logarithmic data.
It is further preferred that the first and second logarithmic conversion devices are one logarithmic conversion device.
It is yet further preferred that the first characteristic value acquiring device is a luminance meter with which a transmitted light from the image display apparatus is measured and that provides luminance data for obtaining the first characteristic values.
According to its fourth aspect, the present invention provides an image display system further comprising an interpolation device for interpolating the respective logarithmic data of the first characteristic values and the second characteristic values, respectively, wherein the gradation correction table constructing device constructs the gradation correction table based on the respective logarithmic data of the first characteristic values and the second characteristics values that have been interpolated, respectively.
In both the third and fourth aspects of the invention, it is preferable that the interpolation device can select the interpolation between high-order interpolation and linear interpolation.
It is also preferably that the interpolation device adapts to switch the interpolation between high-order interpolation and linear interpolation depending upon a number of discrete points to be interpolated.